52 FKAGMENTS OF SCIENCE. 



Hence, to make the candle-light a mile off equal in 

 power to the non-luminous radiation received from the 

 electric light at a foot distance, its intensity would 

 have to be multiplied by 1,500x20,000,000, or by 

 thirty thousand millions. Thus the thirty thousand 

 millionth part of the invisible radiation from the 

 electric light, received by the retina at the distance ot 

 a foot, would, if slightly changed in character, be amply 

 sufficient to provoke vision. Nothing could more 

 forcibly illustrate that special relationship supposed by 

 Melloni and others to subsist between the optic nerve 

 and the oscillating periods of luminous bodies. The 

 optic nerve responds, as it were, to the waves with 

 which it is in consonance, while it refuses to be excited 

 by others of almost infinitely greater energy, whose 

 periods of recurrence are not in unison with its own. 



10. Persistence of Rays. 



At an early part of this lecture it was affirmed, that 

 when a platinum wire was gradually raised to a state 

 of high incandescence, new rays were constantly added, 

 while the intensity of the old ones was increased. Thus, 

 in Dr. Draper's experiments, the rise of temperature 

 that generated the orange, yellow, green, and blue 

 augmented the intensity of the red. What is true of 

 the red is true of every other ray of the spectrum, 

 visible and invisible. We cannot indeed see the aug- 

 mentation of intensity in the region beyond the red, 

 but we can measure it and express it numerically. 

 With this view the following experiment was performed : 

 A spiral of platinum wire was surrounded by a small 

 glass globe to protect it from currents of air ; through 

 an orifice in the globe the rays could pass from the 



